In modern handheld devices for cellular communication systems (e.g. 3GPP) there is a desire to support multiple frequency bands (e.g 3GPP LTE bands 1, 2, 3, 5, 7, 8, and 13). Further in this regard, one particular area of interest in radio communication has been how to deal with transmitter and receiver duplexing, with transmitter and receiver operating at a fixed or variable frequency separation. For such frequency division duplex, an issue known as duplex self-interference can present a problem in the design of such systems. The problem arises from the high power of the transmitter challenging the linearity of the receiver that can be set up to have a high gain to deal with low power reception levels.
For small handheld devices, duplex operation has typically been achieved using fixed frequency filters known as duplex filters (e.g., dielectric coaxial resonator filters, SAW, BAW, FBAR) that are switched among operating frequencies (e.g., for multiband operation) using semiconductor switches due to technology and size constraints. These fixed frequency filters and antennas exhibit particular limitations in that, for each band of operation, a new set of hardware must be introduced (e.g., adding antenna resonator coupling element, filters, and switches when adding band support). As a result, tunable systems would be beneficial in reducing the amount of hardware required to operate at a range of frequencies, but it is difficult to make a tunable system that is also cost effective and small while at the same time meeting system requirements (e.g., 3GPP standards). In addition, although a majority of cellular frequency bands (e.g., 3GPP standard LTE FDD bands 1 to 25) have reception frequencies above transmission frequencies (i.e., “positive” duplex spacing), there are operating bands (e.g., LTE bands 13, 14, 20, and 24) that have the reverse order (i.e., “negative” duplex spacing) such that the reception frequencies are below the transmission frequencies. As a result, designing systems that are able to operate in either kind of spacing presents further impediments to reducing the number of hardware components required.
Accordingly, it would be desirable for there to be a solution to change the characteristics of the filter to accommodate both positive and negative duplex spacing instead of switching between different hardware or filters, which can thereby allow filter resonators to be reused for both positive and negative duplex spacing. In addition, it would be further desirable for a solution to bring down the size of a tunable solution, to make it cost efficient, and at the same time, with proper design, to solve the issue of removing unwanted interference, such as from a transmitter in the wireless communication terminal.